In the state-of-the-art catalysis, design and development of new catalytic systems for selective organic transformations is of significant interest, though very challenging. Particularly, alternative ‘green’ approaches to traditionally employing stoichiometric reagents and/or harsh conditions in industrially important reactions are extremely essential with regard to both energy and environment. The direct conversion of alcohols to value-added products and hydrogenation of carbonyl compounds to the corresponding alcohols and/or amines are benchmark reactions. Alcohol esterification is one of the most important reactions in synthetic organic chemistry and has potential applications including in fragrance, polymer, and pharmaceutical industries. The reaction is typically carried-out by coupling of activated carboxylic acid derivatives with an alcohol and produce stoichiometric acid waste. An ideal and alternative approach would be catalytic acceptorless dehydrogenation of alcohols with the evolution of molecular hydrogen, but homogeneous systems capable of catalyzing dehydrogenation of alcohols are relatively rare.
Catalytic hydrogenation of carboxylic acid derivatives, particularly esters with hydrogen gas, provides a promising alternative approach to conventional reduction methods. The operational simplicity, ‘green’ approach and economic viability are additional advantages and make this method much more attractive. In industry, this process is carried out under heterogeneous conditions using catalysts such as copper chromite at high pressure (200-300 atm) and temperatures (200-300° C.).
Article titled “Novel ruthenium(II) complexes with substituted 1,10-phenanthroline or 4,5-diazafluorene linked to a fullerene as highly active second order NLO chromophores” by Adriana Valore et al. published in Dalton Trans., 2010, 39, 10314-10318 reports Ru(II) complexes with substituted 1,10-phenanthroline or 4,5-diazafluorene.
Article titled “Stepwise Metal-Ligand Cooperation by a Reversible Aromatization/Deconjugation Sequence in Ruthenium Complexes with a Tetradentate Phenanthroline-Based Ligand” by Langer et al. published in Chemistry, 2013; 19(10), pp 3407-14 reports synthesis and reactivity of ruthenium complexes containing the tetradentate phenanthroline-based phosphine ligand 2,9-bis((di-tert-butylphosphino)methyl)-1,10-phenanthroline (PPhenP).
Chinese patent application no. 103980317 discloses a dipyridyl tetradentate ligand ruthenium complex as well as a preparation method of the complex and application of the complex to a reaction for hydrogenating ester compounds into alcohol compounds.
US patent application no. 20130281664 discloses novel ruthenium catalysts and related borohydride complexes, and the use of such catalysts for (1) hydrogenation of amides (including polyamides) to alcohols and amines; (2) preparing amides from alcohols with amines (including the preparation of polyamides (e.g., polypeptides) by reacting dialcohols and diamines and/or by polymerization of amino alcohols); (3) hydrogenation of esters to alcohols (including hydrogenation of cyclic esters (lactones) or cyclic di-esters (di-lactones) or polyesters); (4) hydrogenation of organic carbonates (including polycarbonates) to alcohols and hydrogenation of carbamates (including polycarbamates) or urea derivatives to alcohols and amines; (5) dehydrogenative coupling of alcohols to esters; (6) hydrogenation of secondary alcohols to ketones.
Article titled “Electron-Rich PNP- and PNN-Type Ruthenium (II) Hydrido Borohydride Pincer Complexes. Synthesis, Structure, and Catalytic Dehydrogenation of Alcohols and Hydrogenation of Esters” By J Zhang et al. published in Organometallics, 2011, 30 (21), pp 5716-5724 reports Electron-rich PNP- and PNN-type ruthenium(II) hydrido borohydride pincer complexes, [RuH(BH4)(tBu-PNP)] (tBu-PNP=(2,6-bis(di-tert-butylphosphinomethyl)pyridine) (5) and [RuH(BH4)(tBu-PNN)] (tBu-PNN=2-di-tert-butylphosphinomethyl-6-diethylaminomethylpyridine) (6), prepared from their corresponding N2-bridged dinuclear Ru(II) complexes [(tBu-PNP)RuCl2]2(μ-N2) (3) and [(tBu-PNN)RuCl2]2(μ-N2) (4), respectively.
Article titled “PNN Ruthenium Pincer Complexes Based on Phosphinated 2,2′-Dipyridinemethane and 2,2′-Oxobispyridine. Metal-Ligand Cooperation in Cyclometalation and Catalysis” By Rigoberto Barrios-Francisco et al. published in Organometallics, 2013, 32 (10), pp 2973-2982 reports synthesis of novel PNN ruthenium pincer complexes based on 2,2′-dipyridinemethane phosphine derivatives, as well as on 2,2′-oxobispyridine phosphine ligands, and their reactivity toward dearomatization and cyclometalation.
Article titled “Synthesis of Amides from Esters and Amines with Liberation of H2 under Neutral Conditions” by Boopathy Gnanaprakasam et al. published in J. Am. Chem. Soc., 2011, 133 (6), pp 1682-1685 reports efficient synthesis of amides directly from esters and amines is achieved under mild, neutral conditions with the liberation of molecular hydrogen which is homogeneously catalyzed under neutral conditions by a dearomatized Ru-pincer PNN complex.
PCT application no. 2006106484A1 discloses catalytic hydrogenation and use of Ru complexes with tetradentate ligands, having at least one amino or imino coordinating group and at least one phosphino coordinating group, in hydrogenation processes for the reduction of esters or lactones into the corresponding alcohol or diol respectively.
Article titled “Facile Conversion of Alcohols into Esters and Dihydrogen Catalyzed by New Ruthenium Complexes” by Jing Zhang et al. published in in J. Am. Chem. Soc., 2005, 127, 10840-10841 reports Ru(II) hydride complexes based on electron rich PNP and PNN ligands catalyze alcohol dehydrogenation to esters.
Heterogeneous catalytic systems for direct hydrogenation of CO2 to methanol require high temperature and pressure and often selectivity is the major issue. Owing to rational tuning of reactivity and selectivity of homogeneous catalysts much attention has been paid for their utility in selective hydrogenation of CO2. In recent years, pincer ligands which bind to metal centers in a tri-dentate, meridional fashion have drawn much attention and serve as excellent ligands, due to stability and variability of the generated metal-ligand framework. The donor/acceptor ability at both the central and adjacent side-arm positions of the pincer ligands can be controllable. And both the electronic and steric environment around the metal center can also be tunable.
Article titled “Carbon dioxide hydrogenation to methanol at low pressure and temperature” by Bill Alain published in EPFL, 1998 reports the conversion of CO2 with hydrogen to methanol is investigated in dielectric-barrier discharges with and without catalysts at low temperatures (≤100° C.) and pressures (≤10 bar).
Article titled “Bifunctional catalysis: A bridge from CO2 to methanol” by Pierre H. Dixneuf published in Nature Chemistry, 2011, 3, pp 578-579 reports methanol can be produced via the hydrogenation of carbonates and carbamates using a pincer ruthenium(II) catalyst.
Article titled “Homogeneous hydrogenation of carbon dioxide to methanol” by Yu-Nong Li et al. published in Catalysis Science & Technology, 2014, 4(6), pp 1498-1512 reports metal complexes and organocatalysts for CO2 hydrogenation to methanol have been developed along with the reaction mechanistic insight. Understanding the interaction of active catalytic species with CO2 or hydrogen could account for development of efficient homogeneous catalysts.
Article titled “Hydrogenation of carbon dioxide to methanol using a homogeneous ruthenium-Triphos catalyst: from mechanistic investigations to multiphase catalysis” by Sebastian Wesselbaum et al. published in Chemical Science, 2015, 6, 693-704 reports Hydrogenation of carbon dioxide to methanol using a homogeneous ruthenium-Triphos catalyst: from mechanistic investigations to multiphase catalysis.
Article titled “Hydrogenation of carbon dioxide to methanol by using a homogeneous ruthenium-phosphine catalyst” by Sebastian Wesselbaum et al. published in Angewandte Chemie International Edition, 2012, 51(30):7499-502 reports the homogenously catalyzed hydrogenation of CO2 to methanol is achieved by using a ruthenium phosphine complex under relatively mild conditions (HNTf2=bis(trifluoromethane)sulfonimide). This is the first example of CO2 hydrogenation to methanol by using a single molecularly defined catalyst.
Article titled “Cascade hydrogenation of carbon dioxide to methanol” by Chelsea Ariane Huff published as thesis in 2014 reports use of homogeneous catalysts in tandem for the hydrogenation of CO2 to CH3OH at relatively high temperature (135° C.).
Article titled “Tandem amine and ruthenium-catalyzed hydrogenation of CO2 to methanol” by Rezayee N M et al. published in Journal of American Chemical Society, 2015, 28; 137(3), pp 1028-31 reports the hydrogenation of carbon dioxide to methanol via tandem catalysis with dimethylamine and a homogeneous ruthenium complex. The dimethylamine is proposed to play a dual role in this system. It reacts directly with CO2 to produce dimethylammonium dimethylcarbamate, and it also intercepts the intermediate formic acid to generate dimethylformamide. With the appropriate selection of catalyst and reaction conditions, >95% conversion of CO2 was achieved to form a mixture of CH3OH and dimethylformamide.
Article titled “Examining ruthenium chromophores for the photochemical reduction of CO2 to methanol” by David J Boston published as thesis in 2013 reports photochemical reduction of carbon dioxide to methanol is possible using pyridine and the complex [Ru(phen)2dppz](PF6)2, [Ru(phen)2pbtpα](PF6)2, and [Ru(phen)2pbtpβ](PF6)2.
Article titled “Cascade catalysis for the homogeneous hydrogenation of CO2 to methanol” by Chelsea A. Huff et al. published in Journal of American Chemical Society, 2011, 133 (45), pp 18122-18125 reports the homogeneous hydrogenation of CO2 to CH3OH via cascade catalysis. Three different homogeneous catalysts, (PMe3)4Ru(Cl)(OAc), Sc(OTf)3, and (PNN)Ru(CO)(H), operate in sequence to promote this transformation.
Article titled “Efficient hydrogenation of organic carbonates, carbamates and formates indicates alternative routes to methanol based on CO2 and CO” by Balaraman E et al. published in Nature Chemistry, 2011, 3(8), pp 609-14 reports catalytic hydrogenation of organic carbonates to alcohols, and carbamates to alcohols and amines. Unprecedented homogeneously catalysed hydrogenation of organic formates to methanol has also been accomplished. The reactions are efficiently catalysed by dearomatized PNN Ru(II) pincer complexes derived from pyridine- and bipyridine-based tridentate ligands.
Article titled “Unprecedented catalytic hydrogenation of urea derivatives to amines and methanol” by Dr. Ekambaram Balaraman et al. published in Angewandte Chemie International Edition, 2011, 50(49):11702-11705 reports hydrogenation of urea derivatives to the corresponding amines and methanol is reported. The reaction is catalyzed by a bipyridine-based tridentate PNN Ru(II) pincer complex and proceeds under mild, neutral conditions using 13.6 atm of H2. A mild approach is offered for the indirect hydrogenation of CO2 to methanol as urea derivatives are available from CO2.
Article titled “Conversion of CO2 from air into methanol using a polyamine and a homogeneous ruthenium catalyst” by Jotheeswari Kothandaraman et al. published in Journal of American Chemical Society, 2015 reports a highly efficient homogeneous catalyst system for the production of CH3OH from CO2 using pentaethylenehexamine and Ru-Macho-BH (1) at 125-165° C. in an ethereal solvent has been developed (initial turnover frequency=70 h−1 at 145° C.).
Article titled “How does the nickel pincer complex catalyze the conversion of CO2 to a methanol derivative? A computational mechanistic study” by Huang F et al. published in Inorganic Chemistry, 2011, 18; 50(8), pp 3816-25 reports the mechanistic details of nickel-catalyzed reduction of CO2 with catecholborane (HBcat) have been studied only by DFT calculations. The nickel pincer hydride complex ({2,6-C6H3(OP(t)Bu2)2}NiH═[Ni]H) has been shown to catalyze the sequential reduction from CO2 to HCOOBcat, then to CH2O, and finally to CH3OBcat.
However, there is a need in the art to develop novel catalyst for an atom-economical, environmentally benign and operationally simple synthetic process from the feedstock chemicals for the direct conversion of alcohols to esters and efficient hydrogenation of esters to alcohols under solvent-free reaction conditions.